Small outbuilding foundation where it's wet and windy - pier & beam or slab?

[sniederb]

I'm planning to put up a bird hide in our back yard, Western Isles (Scotland). The structure itself will be timber, approx. 3m x 2m and 2m high, slanted roof (maybe with solar panels). The structure doesn't need insulation, but damp and rot is a concern (did I mention Western Isles?) The ground seems to be a mix of gravel and soil about 50cm deep, then solid rock. (The image shows roughly the intended size, using some spare clay liners to indicate the corners)

 

I'm struggling to decide on the type of foundation. We're quite a bit away from the road, so the amount of concrete needed is fairly relevant.

 

My understanding so far is that I could do pier & beam, which would mean to dig 4 holes, use some sort of sonotube and put in beam connectors. The alternative would be a monolithic slab, which would mean digging the entire area down about 20cm, add a frame on top and then fill it all with concrete.

The pier & beam option is attractive because it needs so much less concrete, but what I read on this forum is that most people put down a slab.

 

What are the things I need to consider to reach a decision?

[MikeSharp01]

I was thinking just yesterday about how to provide foundation for our lean too shed on the garden room for batteries, the solar panels are on the roof of the garden room. My analysis was that the whole decision came down to permanency and how much and material I would need, there seemed like three options. Slab, Post and Concrete beams with topping of screed, Post and Wooden Beam (which I assume is what you are considering - but I may have that wrong). In the end I decided to go for the concrete slab approach as it is only a small area (1m²) and will give me a simple install but I can wheel barrow the concrete in, albeit with difficulty. The alternative concrete beam approach was expensive and complex for such a small space and the wooden approach, although the simplest was not the most enduring. 

 

So I am going for concrete slab but with your bigger space and difficult access I would go for the wooden post & beam approach and make sure you have good ventilation below to keep it as dry as you can.

[Conor]

The 3x2m timber structure you describe is basically a large shed. Assume no need for running water, tiled floor etc? Basically box, opening on one side and a bench seat or similar? It's not going to weigh much. You could get away with putting it on timber stilts, sitting on paving slabs. Or even on a trailer chassis if you ever need to move it. No concrete needed either way.

Edited January 13 by Conor

[sniederb]

5 minutes ago, Conor said:

The 3x2m timber structure you describe is basically a large shed. Assume no need for running water, tiled floor etc? Basically box, opening on one side and a bench seat or similar? It's not going to weigh much. You could get away with putting it on timber stilts, sitting on paving slabs. Or even on a trailer chassis if you ever need to move it. No concrete needed either way.

Yes, it's really just a wooden box with some chairs in it. Maybe a place for the tea kettle, if I get that to work with solar. I was thinking to pour 4 concrete piers with beam connectors on top, I fear timber stilts will rot too quickly. But from your reply I gather that those 4 concrete pier will easily do the job.

[JohnMo]

My pallet shed is just on concrete blocks levelled. Our summer house is on heavy concrete 3x2 slabs.

 

Or do screw piles - no concrete

[AartWessels]

40 minutes ago, sniederb said:

Yes, it's really just a wooden box with some chairs in it. Maybe a place for the tea kettle, if I get that to work with solar. I was thinking to pour 4 concrete piers with beam connectors on top, I fear timber stilts will rot too quickly. But from your reply I gather that those 4 concrete pier will easily do the job.

I've built a shed slightly bigger on 6 concrete piers of 400mm high. Dug them in, and put 2 4x4 beams in parallel on top of them.

Then, built the floor frame on top of these beams. It gives some ventilation underneath to allow things to dry. I've built a large decking in more or less the same way. Saved me from hammering in hundreds of stakes 😂

[Conor]

59 minutes ago, sniederb said:

Yes, it's really just a wooden box with some chairs in it. Maybe a place for the tea kettle, if I get that to work with solar. I was thinking to pour 4 concrete piers with beam connectors on top, I fear timber stilts will rot too quickly. But from your reply I gather that those 4 concrete pier will easily do the job.

I don't think you need to pour anything. A stack of pavers or concrete block to a couple hundred mm above the ground. Maybe one in each corner and a couple on the long length. Then just set the timber base on top. If it's exposed you'll want some sort of tether sunk in to the ground. 

[sniederb]

30 minutes ago, Conor said:

I don't think you need to pour anything. A stack of pavers or concrete block to a couple hundred mm above the ground. Maybe one in each corner and a couple on the long length. Then just set the timber base on top. If it's exposed you'll want some sort of tether sunk in to the ground. 

Conor, I'm worried about the strong winds we get. So the "tethers sunk in to the ground" is why I'm thinking to pour concrete piers.

[sniederb]

51 minutes ago, AartWessels said:

I've built a shed slightly bigger on 6 concrete piers of 400mm high. Dug them in, and put 2 4x4 beams in parallel on top of them.

Then, built the floor frame on top of these beams. It gives some ventilation underneath to allow things to dry. I've built a large decking in more or less the same way. Saved me from hammering in hundreds of stakes 😂

Cool, that sounds pretty much exactly like what I have in mind.

[Galileo]

On 13/01/2024 at 09:32, sniederb said:

Conor, I'm worried about the strong winds we get. So the "tethers sunk in to the ground" is why I'm thinking to pour concrete piers.

I was thinking the exact same thing, if the Western isles are anything like Shetland, if it's not fixed firmly to mother earth it will make its way to Norway rapidly.

 

Built a storage shed on my croft back end of last year, made concrete piers formed with an upturned plastic bucket with the bottom cut off, cardboard concrete formers are not common/cheap in the UK for some reason. I used 6 piers, so I could save on timber thickness for the beams and use what the builders merchants had in stock, getting chunky Douglas Fir off the shelf is not an option up here.

[kandgmitchell]

A few years ago in order to build a deck over very soft wet ground I set short lengths of 4" x 4" treated fence posts into the ground surrounded by post mix. Once cut to the right height I then coach bolted 7" x 2" joists to the faces to make a frame, infilled with smaller joists and decked over.

 

Could you use the same system and just frame your walls off the deck/floor so the hide is essentially sat on timber piles which provide support and anchorage.